(1) Field of the Invention
The present invention relates to a marine contra-rotating propeller apparatus which includes an outside propeller for propelling a ship and an inside propeller which is rotated in the opposite direction to that of the outside propeller at the same revolution speed as that of the outside propeller.
(2) Description of the Prior Arts
A conventional marine contra-rotating propeller apparatus is now described, by way of example, with reference to FIG. 1. The marine contra-rotating propellers are composed of the combination of a stern-side or outside propeller 1 and a bow-side or inside propeller 2. The torque delivered by a main diesel engine 7 is transmitted to a planetary gear mechanism 5 through an elastic coupling 6 which eliminates any variation component or vibration component of the torque. The torque transmitted to the planetary gear mechanism 5 rotates an inner shaft 4 through a spur gear meshing with a planet gear and the revolution of the planet gear is taken out from an outer shaft 3 so that the outer shaft 3 is rotated in the opposite direction to that of the inner shaft 4 and a substantially equal torque is transmitted to both the shafts 3 and 4. The propellers 1 and 2 are rotated in the opposite directions to each other at the substantially same revolution speed depending on the design of a shape of the propellers. In other words, the contra-rotating propellers are designed to be rotated in the opposite directions to each other with the torque and the revolution speed being substantially equal and with the high efficiency of propulsion.
The thrust developed by the outside propeller 1 and the inside propeller 2 is transmitted through the inner shaft 4 and the outer shaft 3 to a ship body by means of a thrust bearing 8. The elastic coupling 6 smooths the variation component of the torque and serves to protect the gears of the planetary gear mechanism 5 for reversing the revolution direction.
However, while the conventional marine contra-rotating propeller apparatus is designed so that the revolution directions of both the propellers are opposite and the revolution speed and the torque are substantially equal in order to optimize the efficiency of propulsion, the adoption of the planetary gear mechanism shown in the figure produces the following drawbacks:
(I) If both the propellers 1 and 2 of the contra-rotating propeller apparatus are rotated in the opposite directions to each other at the same revolution speed using the planetary gear mechanism 5, the number of revolutions of the main diesel engine is reduced (the reduction ratio is 3 or more) to be transmitted to the propellers. Accordingly, if a low-speed diesel engine having an output shaft rotated at a low revolution speed (for example, 70 revolutions/minute) is used, the revolution of the propellers is reduced below a desired optimum revolution speed and the efficiency of propulsion is deteriorated. If a middle-speed diesel engine having an output shaft rotated at a relatively high revolution speed (for example, 450 revolutions/minute) is used, it can not obtain the advantages of the low-speed diesel engine such as good fuel consumption rate, easiness in maintenance and usability of bad quality fuel although the revolution of the propellers can be set near a desired maximum revolution speed and the improved efficiency of propulsion can be expected.
Further, since the speed reduction ratio of the input side and the output side, that is, the outer shaft 3 and the inner shaft 4 and the torque ratio of the outer shaft 3 and the inner shaft 4 in the planetary gear mechanism 5 are related to each other, it is necessary that the main diesel engine be provided at its output shaft with a separate speed reduction or increase device to adjust the revolution of the planetary gear mechanism at its input side so that the torque ratio is optimized. (Since the main diesel engine possesses a rated output power and revolution speed, a continuous revolution speed can not be selected.)
(II) Recently, a main engine in a large ship mainly uses a low-speed diesel engine due to the above advantages. Since the low-speed engine is directly coupled to the propeller through a shaft generally, the main engine contains a thrust bearing therein in a standard configuration. If the planet gear is disposed between the main diesel engine and the propeller, the thrust bearing is required to be provided between the propeller and the planet gear. Consequently, the thrust bearing in the main engine merely serves to stop the movement of a crank shaft of the main diesel engine and the capacity thereof is vainly too large. (In FIG. 1, the thrust bearing 8 receives the thrust of the propellers 1 and 2 while the thrust bearing (not shown) in the main engine does not receive the thrust of the propellers.)